Fix Backend TLS documentation

[zigarn]

Description

The current Backend TLS documentation is broken on multiple levels:

• the nginx manifest is from gloo repository instead of kgateway one, despite the description saying so
• the applied manifests (nginx and CA cert) have a specified namespace that is never created in the documentation
• the CA cert is broken in the rendered page because the templating engine is escaping some chars (like + becoming +)

This PR fixes all that:

• Update nginx manifest URL for kgateway repository instead of gloo
• Create and use the right namespace defined in the manifests
• Apply public CA certificate ConfigMap directly from URL as the templating is escaping some chars and breaking the certificate when trying to inject content directly

Change Type

/kind documentation
/kind fix

Changelog

NONE

Additional Notes

[zigarn]

/kind documentation
/kind fix